1. Field of the Invention
This invention relates to a system for enabling communication between an underwater fluid extraction facility and a remote location.
2. Description of Related Art
Known underwater fluid extraction and production systems typically utilise a bus communication system to pass control signals between a remotely located control station, for example a surface station, and an underwater facility, for example a subsea hydrocarbon extraction field. In such systems, control modules at the facility are connected to a communications umbilical in a multi-drop arrangement, the umbilical leading to the surface station. This has worked well in so far, as the communications load has been limited to, for example, opening or closing a valve at the facility occasionally and reporting temperatures, pressures and other simple measurements, typically once per minute. However, with the advent of more sophisticated sensors the multi-dropped bus system is no longer adequate.
Typically a subsea fluid extraction field is split up into a number of templates, each supporting a group of wells. In traditional production control systems, each template is operated via its own dedicated circuit in the umbilical cable bundle, running with a data rate between 1.2 and 19.2 Kbps. Currently there is a tendency for offshore well fields to be a substantial distance away from the control station, typically greater than 100 km, with communication being effected via an umbilical housing fibre optic cables. The fibre optic link between the shore and the field typically runs at a minimum of 4 Mbps and may be as fast as 2.49 Gbps. It therefore makes sense to combine the circuits, by multiplexing, into a single high data rate channel to take advantage of the higher link speed.
There are a number of approaches to multiplexing, but in existing systems, typically, each circuit has a dedicated channel which is allocated a specified time slot in a time division multiplexing system. This is known as a ‘virtual’ channel. Typically the ports are taken to a router which multiplexes their data to a single high rate signal which is passed to a ‘long haul’ optical modem. At the remote end, the signal is demultiplexed to reclaim the individual channels, which are then passed to modems for onward transmission to the appropriate template.
FIG. 1 illustrates a typical ‘virtual channel’ system, whereby a shore-based master control station (MCS) 1 connects to a data router and packetiser 4 via RS 422 interfaces 2 and 3. The packetiser 4 includes a fibre optic modulator driving an optical fibre 5. The modulated light signal is converted back to an electrical signal at the other end of the fibre 5, at a second data router and packetiser 6. This outputs digital packages via RS 422 interfaces 7 to high-speed copper modems (HSCM) 8, and then onto the appropriately addressed templates. Each template is connected to a number of subsea electronic modules (SEM) mounted on well trees (not shown in FIG. 1), which thus receive control data and return monitoring data.
The virtual channels created are full duplex, independent and isolated from each other. Communication signals at the MCS in this system cannot be distinguished from those in a system where the MCS drives the modems locally, so it provides a way of concentrating a number of long-haul signals onto a single fibre while using conventional subsea distribution techniques and protocols. The virtual channel approach was developed to support the conventional production control system architecture of a MCS communicating with a group of well tree mounted SEMs, multi-dropped on a single umbilical. Each of the systems is allocated the same bandwidth and third party sensor equipment is either integrated into the control system or a transparent channel is provided to allow communication between the sensor and its topside control unit. It has served the industry well in the past but subsea sensor systems are becoming more intelligent and demanding sufficient bandwidth to justify direct connection to the communications distribution network. Typical of this increased demand is the requirement to include real time video in the data stream.